Scar has shown pics of his Amoeba lights mounted on the sides of a FF helmet. Trailtech make a mount for their lights on the side of a FF moto helmet.

Getting the light off the top of the helmet makes loads of sense depending on where you ride. In my area the rain laden fir tree branches droop lower in winter than summer waiting to snag a high mounted light. Getting an old NR halogen yanked off the top of my helmet is what pushed me to go with a low, forward mounted helmet light.

My issue with lights on the side, is the farther you get off of center, the greater the leverage, increasing the apparent weight and strain on the neck as well as helmet play/wobble. Other than that no real reason not to I guess.